Iron, Silver and Copper are all metal. Only Oxygen in this specific choices is not metal. Oxygen is known to be the 3rd most abundant element in the Universe but the 1st most abundant element on earth by mass. And we all know that Oxygen is a very essential part in our daily living in this world, actually not only humans who need Oxygen but also some creatures like animals. Oxygen is represented as O in the periodic table and its atomic number is 8.